1. Field of the Invention:
The present invention relates to a land vehicle navigation system using a global positioning system (GPS). The land vehicle navigation system has the function that the optimum combination of GPS satellites (each hereinafter referred to "satellite" simply) can be selected to fixedly determine the present position of the vehicle in urban areas which include many buildings tending to block signals from satellites.
2. Description of the Prior Art:
The global positioning system can fixedly determine the present position of the vehicle by processing signals transmitted from preselected three or four satellites.
On selection of three or four satellites, the general method is to select such a combination of satellites that increases an accuracy of position determination as far as possible, that is, to select such a combination minimizing the position dilution of precision (PDOP) which represents radial range errors in the three-dimensional coordinates or the horizontal dilution of precision (HDOP) which represents radial range errors in the two-dimensional coordinates.
When a vehicle runs in an urban area or in a hilly country, the signals from satellites are frequently blocked by the buildings or mountains. Therefore, even if a combination of satellites is selected to minimize the PDOP or HDOP, the vehicle navigation system could be utilized only in very limited areas.
To improve the above problem, Japanese Laid-Open patent application 61-198074 has proposed to provide a memory section for storing map and terrain data with respect to an area in which the vehicle is to run. The memory section further stores necessary data for selecting the optimum combination of the satellite. For determinating which area the vehicle is running, the data includes suburban or urban indications on the map. The mountain height and position data is also available to exclude satellites from which signals might be blocked by the mountain when the vehicle is travelling in suburban area. For urban area driving, the data has to include the information (1) to select satellites having as much higher elevation angle as possible at the vehicle position user, (2) to select satellites each having its azimuth angle being coincident with the travelling direction of the vehicle or with the opposite direction thereof and (3) to exclude any satellite from which signals are frequently not received by ambiguous reason. However, the optimum satellite combination can not be always selected in accordance with the distribution of building height in the urban area using the above method.